This invention relates to the field of information handling systems and specifically to second level packaging in which chip carrier modules are attached to circuit boards and more specifically to surface mount attachment of ceramic modules to organic circuit boards.
In the related field of first level packaging solder connections have been used for mounting ICs (integrated computer chips) using the C-4 (controlled collapse chip connection) technology since first described in U.S. Pat. Nos. 3,401,126 and 3,429,040 by Miller. Packaging Electronic Systems by Dally ( McGraw-Hill 1990 p. 113) describes flip chip or C-4 connections. In Dally, xe2x80x9cChip bond pads are deployed in an area array over the surface of the chip. . . . These bonding pads are 5 mil in diameter on 10 mil centers. Matching bonding pads are produced on a ceramic substrate so that the pads on the chip and the ceramic coincide. Spheres of solder 5 mil in diameter are placed on the ceramic substrate pads . . . and the chip is positioned and aligned relative to the substrate. The assembly is heated until the solder spheres begin to soften and a controlled collapse of the sphere takes place as the solder simultaneously wets both pads. A myriad of solder structures have been proposed for mounting IC chips as well as for interconnection to other levels of circuitry and electronic packaging.xe2x80x9d Ceramic is used as a substrate because it has a thermal coefficient of expansion (TCE) which is close to that of the silicon IC chips.
In the field of second level packaging xe2x80x9cBall Grid Arrays: The Hot New Packagexe2x80x9d by Terry Costlow and xe2x80x9cSolder Balls Make Connectionsxe2x80x9d by Glenda Derman both in Electronic Engineering Times Mar. 15, 1993, describe using solder balls to connect ceramic or flexible chip carriers to circuit boards.
U.S. Pat. No. 4,132,341 to Bratschum describes the self-centering action of conductors spanning between solder pads of two components when both pads are simultaneously reflowed. U.S. Pat. No. 4,831,724 describes the self-centering of a component when it is vibrated during reflow.
Fabrication of multi-layer ceramic chip carriers is described in U.S. Pat. Nos. 3,518,756; 3,988,405; and 4,202,007 as well as xe2x80x9cA Fabrication Technique For Multi-Layer Ceramic Modulesxe2x80x9d by H. D. Kaiser et al., Solid State Technology, May 1972, pp. 35-40 and xe2x80x9cThe Third Dimension in Thick-Films Multilayer Technologyxe2x80x9d by W. L. Clough, Microelectronics, Vol. 13, No. 9 (1970), pp. 23-30.
Fabrication of multi-layer circuit boards is described in U.S. Pat. Nos. 3,554,877; 3,791,858; and 3,554,877. Thin film techniques are described in U.S. Pat. No. 3,791,858.
U.S. Pat. No. 4,604,644 to Beckham describes materials and structures for encapsulating C-4 connections. U.S. Pat. No. 4,701,482 to Itoh and U.S. Pat. No. 4,999,699 to Christie et al. disclose epoxies and guidance in selecting epoxies for electronic applications.
Flexible film chip carriers (known in the art as ATAB or TAB-BGA) are described in U.S. Pat. Nos. 4,681,654; 4,766,670 and 5,159,535. In ATAB (area tape automated bonding) a flexible circuit board chip carrier is mounted on a circuit board using solder-ball connect.
U.S. Pat. No. 5,147,084 to Behun, describes using a HMP (high melting point) solder ball in association with a LMP (low melting point) solder. FIG. 1A of that patent is similar to FIG. 4 of this application. xe2x80x9cA part 10 is to be joined to a board 11. Part 10 has internal metallurgy 14 which terminates at the surface at a bonding pads 12. A . . . LMP solder 16 is applied to a bonding pad 12. A . . . HMP solder ball 18 is placed in contact with LMP solder 16 and the assembly is heated to reflow the LMP solder which then wets to the non-molten HMP solder ball. . . . Board 11 is also illustrated with internal metallurgy 15, terminating on the surface bonding pad 17 . . . the assembled part 10 . . . is brought into contact with part 11 having pad 17 and LMP solder 13, and the two are heated to a temperature sufficient to reflow the LMP solder but not sufficient to melt the HMP solder ball. The LMP solder 13 which is attached to the bonding pad 17, on board 11, will wet the HMP ball and connection will be achieved.xe2x80x9d
All the above sources are hereby incorporated by reference.
In the invention of applicants, high melting temperature (HMT) preforms (balls or columns) connect between a grid array of contacts on the bottom of a component and a mirror image grid array of contacts on the top surface of an organic substrate to form an electrical interconnect structure. Low melting temperature (LMT) joining material connects between the HMT preforms and the contacts. The invention includes the compositions of the HMT preforms and LMT joining material, the specific geometry of the connections, organic circuit boards and ceramic chip carriers for such interconnections, the process for producing such boards and carriers, and the process for attaching the carriers to the boards. In this description xe2x80x9csolder-ball connectionxe2x80x9d refers to using LMT joining materials to mechanically and electrically connect HMT preforms on a component to conductive contacts on a substrate.
Therefore, it is an object of this invention to provide a process for manufacturing a reliable interconnect assembly using HMT solder-ball connection.
More specifically, it is an object to connect two rigid, confronting substrates using HMT solder-ball connections to form an electronic packaging structure.
It is another object of this invention to provide a method of reflow soldering to produce solder-ball connections.
It is another object of this invention to provide methods for producing a component for solder-ball connection.
It is another object of this invention to provide methods of positioning solder-balls on such a component and reflow joining solder-balls to the component for use in solder-ball connection.
It is another object of this invention to provide a method of selecting HMT solder-ball size, selecting contact size, and selecting LMT solder volume.
It another object of this invention to provide a method of producing metal contacts on substrates for solder-ball connection.
Furthermore, it is an object of this invention to provide a reliable interconnect assembly in which HMT metal-balls are connected between mirror image arrays of contacts of two rigid, confronting substrates.
It is more specifically an object of this invention to define reliable LMT solder joint configurations between the balls and contacts.
It is another object of this invention to define ball sizes and contact sizes required for reliable connection.
It is another object of this invention to define HMT ball materials and LMT solder materials which permit reliable connections to be made.
It is another object of this invention to define substrates which may be used for reliable HMT solder-ball connection.
It is another object of this invention to define structures in a surface wiring layer of a substrate to connect between PTHs (plated through-holes) and connection pads for controlling LMT solder volumes for the joints between the pads and HMT solder-balls.
Finally, it is an object of this invention to describe an information handling system using the connections of the system.
In this invention of applicants, it was discovered that solder-ball connections between confronting metal contact grids on rigid substrates which were made using a process similar to that which was used for ATAB were not reliable due to thermal fatigue of the solder joints between the balls and the contacts. It was discovered that the joints were not all symmetrical due to mis-registration of contacts (allowable tolerances in contact location) causing misalignment between confronting contacts, and that the joints could be made more symmetrical and more reliable by simultaneously reflowing the top and bottom LMT solder joints between each HMP metal-ball and both respective contacts of the ball. This allows the balls to be moved by surface tension of the melted solder to more symmetrical positions between the centers of the contacts within the plane defined by the array of the solder-balls.
It was discovered that making the balls larger reduces fatigue, but that the size of the balls is constrained by the specified interconnection spacing and a nominal spacing between balls necessary to reliably prevent electrical connection from developing between the balls. Similarly, it was discovered that making the contacts larger reduces fatigue, but the size of the contacts are constrained by the specified interconnect spacing and the nominal spacing between contacts necessary to reliably prevent electrical connection from developing between contacts (e.g. solder bridging). For reliable interconnections fatigue is minimized by making the balls slightly smaller than the spacing between contacts and making the contacts slightly smaller than the balls. It was discovered that the reliability of the connections were affected by the relative size between the contacts on either side of each ball and that fatigue could be minimized by making the contacts equal sized. It was discovered that fatigue could be minimized, for different sized contacts on each side of the ball, by making the solder volume larger for the joint with the smaller contact.
It was discovered that increasing the cross section of the solder joints reduced fatigue but the volume increase is constrained by the necessity to reliably prevent solder bridging from developing between adjacent balls and between adjacent contacts. Finally it was discovered that reducing the cross section of the solder joints below about ⅔ of the diameter of the ball has a remarkably deleterious effect on the fatigue life of the connection.